Skin structure and mode of action of vesicles.

The natural function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. Therefore, in the first part of this paper, the barrier function has been explained, focusing on the lipid composition and organisation. The major obstacle for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Several methods have been assessed to increase the permeation rate of drugs temporarily. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper, the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state, liquid-state, and elastic vesicles.